1. Field of the Invention
The present invention relates to a method of manufacturing a piezoelectric ceramic device used for a piezoelectric resonator, a piezoelectric actuator, a piezoelectric filter, a piezoelectric buzzer, a piezoelectric transformer, etc. Particularly, the present invention relates to a method of manufacturing a piezoelectric ceramic device comprising internal electrodes each comprising an alloy composed of Ag as a main component.
2. Description of the Related Art
It is conventionally known that burning under optimum conditions is necessary and essential for a piezoelectric ceramic device used for a piezoelectric resonator, a piezoelectric actuator, a piezoelectric filter, a piezoelectric buzzer, a piezoelectric transformer, etc. in order to have no structural defects, high reliability and excellent part characteristics according to a desired application. Therefore, various attempts to achieve this result have been made so far.
For example, Japanese Unexamined Patent Application Publication No. 2-74566 discloses a burning process under conditions in which the oxygen concentration in a furnace atmosphere is kept at 50% by volume or more in a step of heating to a burning temperature, and the oxygen concentration of the furnace atmosphere in a step of retention at the burning temperature is xc2xd of that in the heating process or 10% by volume. In the heating process under the condition of a high oxygen atmosphere having an oxygen concentration of 50% by volume or more in this process, the oxygen concentration in the ceramic closed pores formed in the heating process is increased. In the next step of retention at the burning temperature, the oxygen concentration of the furnace atmosphere is xc2xd of that in the heating process or 10% by volume, and thus a difference between the oxygen concentrations in the ceramic closed pores and in air is increased to increase the oxygen diffusion rate, thereby permitting dense sintering of piezoelectric ceramic composed of an oxide with high productivity.
Also, Japanese Unexamined Patent Application Publication Nos. 4-357164 and 10-95665 disclose a total burning profile comprising a burning process under atmospheric conditions in which the oxygen concentration is 80% by volume or more. This process can produce a piezoelectric ceramic device, particularly a piezoelectric resonator, which has compactness and high reliability, and which comprises a ceramic sintered body inhibited from producing pores and structural defects.
Furthermore, a total-burning profile comprising a burning process in air, i.e., in an atmosphere of an oxygen concentration of about 21% by volume, regardless of the type of the internal electrodes used is known. This process is generally used for various piezoelectric ceramic parts.
However, the above-described conventional burning processes used for piezoelectric ceramic devices have a problem in that when a green laminate comprising internal electrodes comprising an alloy composed of Ag as a main component is burned, the Ag contained in the internal electrodes is taken into ceramic grains in the piezoelectric ceramic layer, deteriorating piezoelectric characteristics.
Accordingly, it is an object of the present invention to provide a method of manufacturing a monolithic piezoelectric ceramic device exhibiting excellent piezoelectric characteristics, and high reliability.
In order to achieve the object of the present invention, a method of manufacturing a monolithic piezoelectric ceramic device comprises the steps of coating a conductive paste containing an alloy composed of Ag as a main component on a plurality of ceramic green sheets each containing a piezoelectric ceramic material, laminating the plurality of ceramic green sheets to form a laminate, and burning the laminate in an atmosphere in which the oxygen concentrations in the heating process and retention process are about 21% by volume or more, and the oxygen concentration in a cooling process is about 0.05% by volume to about 3% by volume.
The reason for limiting the oxygen concentration in the cooling process during burning to about 0.05% to 3% by volume is as follows.
When burning in an atmosphere containing 0.05% by volume to 3% by volume of oxygen, Ag which dissolved in ceramic grains in the heating process and retention process precipitates at the grain boundaries due to a decrease in the oxygen concentration in the cooling process to cause a state close to the piezoelectric ceramic state before dissolution, and thus the Ag functions to prevent deterioration in piezoelectric characteristics and reliability, thereby improving the piezoelectric characteristics and reliability.
However, when the oxygen concentration is over about 3% by volume, the effect of precipitating Ag, which has been taken in the ceramic grains as described above, in the ceramic grain boundaries is not sufficiently exhibited, thereby failing to improve the piezoelectric characteristics and reliability.
On the other hand, with an oxygen concentration of less than about 0.05% by volume, precipitation of a heterogeneous phase is observed on the piezoelectric ceramic surfaces, and the heterogeneous phase inhibits the piezoelectric characteristics and reliability, thereby failing to improve the piezoelectric characteristics and reliability.
The reason for limiting the oxygen concentrations in the heating process and retention process during burning to about 21% by volume or more is as follows.
With an oxygen concentration or less than about 21% by volume, sufficient sinterability cannot be obtained, which causes deterioration in characteristics.
In the heating process and retention process, for example, a lead zirconate titanate (PZT)-type compound is burned in air having an oxygen content of about 21% by volume, or an atmosphere having an oxygen content of about 21% by volume or more. The burning temperature in the maximum temperature region is set to a temperature at which the laminate is completely sintered in these atmospheric conditions, and specifically, a temperature at which the pack density of the laminated sintered body is 99% or more of the theoretical density. The retention time in the maximum temperature region is also set based on the same idea as the burning temperature.
With the lead zirconate titanate (PZT)-type compound, for example, each of the heating rate and the cooling rate is set in the range of about 1 to 110xc2x0 C./min, and the retention time in the maximum temperature region is set in the range of about 1 to 10 hours.
However, the burning conditions are not limited to the above conditions, and the optimum burning temperature, heating rate, retention time and cooling rate may be selected according to the type (composition system) of the piezoelectric ceramic material.
Besides the lead zirconate titanate (PZT)-type compound, the piezoelectric ceramic material contained in the ceramic green sheets may contain a lead titanate (PbTiO3) compound, a lead metaniobate (PbNb2O6) compound, a lead metatantalate (PbTa2O6) compound, or the like.
The conductor contained in the conductive paste for internal electrodes is composed of Ag as the main component, and can, for instance, contain up to 50% by weight of Pd.
In the method of manufacturing the monolithic piezoelectric ceramic device of the present invention, the internal electrodes each containing an alloy composed of Ag as the main component and the ceramic layers are simultaneously burned under the atmospheric conditions in which the oxygen concentration in the heating process and retention process are about 21% by volume or more, and the oxygen concentration in the cooling process is about 0.05% to 3% by volume. Therefore, Ag taken in grains in the heating process and retention process during burning precipitates in the grain boundaries during the cooling process to improve the piezoelectric characteristics which deteriorate due to Ag uptake, thereby improving the piezoelectric characteristics, particularly a piezoelectric strain constant (d), and sufficiently securing high reliability.
Therefore, a monolithic piezoelectric ceramic device further excellent in the piezoelectric characteristics and reliability can be obtained.
In order to achieve the object, a method of manufacturing a monolithic piezoelectric ceramic device in another aspect of the present invention comprises the steps of coating a conductive paste containing an alloy composed of Ag as a main component on a plurality of ceramic green sheets each containing a piezoelectric ceramic material, laminating the ceramic green sheets to form a laminate, first burning (referred to as xe2x80x9cmain burningxe2x80x9d hereinafter) the laminate, and again burning (referred to as xe2x80x9cre-burningxe2x80x9d hereinafter) the burned laminate in an atmosphere in which the oxygen concentration is about 10% by volume or less.
The reason for limiting the oxygen concentration in the re-burning step to about 10% by volume or less is that with an oxygen concentration of over about 10% by volume, the effect of precipitating Ag, which has been taken in the ceramic grains in the main burning step, in the ceramic grain boundaries is not sufficiently exhibited, thereby failing to improve the piezoelectric characteristics and reliability. On the other hand, the lower limit of the oxygen concentration in re-burning is not limited. With an oxygen concentration of 0.05% by volume, an effect can still be obtained, and therefore, the oxygen concentration may be further decreased according to the burning conditions.
In the main burning step, for example, a lead zirconate titanate (PZT)-type compound is burned in air having an oxygen content of about 21% by volume, or an atmosphere having an oxygen content of about 21% by volume or more. The burning temperature in the maximum temperature region is set to a temperature at which the laminate is completely sintered in these atmospheric conditions, and specifically, a temperature at which the pack density of the sintered laminated body is 99% or more of theoretical density. The retention time in the maximum temperature region is also set based on the same idea as the burning temperature.
After retention in the maximum temperature region in the main burning step, the temperature may be decreased to about 300xc2x0 C. or less, or room temperature, and then the re-burning step may be carried out.
For example, each of the heating rate and the cooling rate in the main burning step is set in the range of about 1 to 110xc2x0 C./min, and the retention time in the maximum temperature region is set in the range of about 1 to 10 hours with the lead zirconate titanate (PZT)-type compound.
In the re-burning step, for example, the lead zirconate titanate (PZT)-type compound is burned in an atmosphere having an oxygen content of about 10% by volume or less. The burning temperature in the maximum temperature region is set to be lower than the burning temperature in the maximum temperature region in the main burning step. For example, the burning temperature is set to be about 20xc2x0 C. to 50xc2x0 C. lower than the top temperature in the main burning step with a lead zirconate titanate (PZT)-type compound. The reason for this is that deterioration in the piezoelectric characteristics due to Pb evaporation is prevented.
With the lead zirconate titanate (PZT)-type compound, each of the heating rate and the cooling rate in the re-burning step is set, for example, in the range of about 1 to 10xc2x0 C./min, and the retention time in the maximum temperature region is set in the range of about 1 to 10 hours.
However, the burning conditions for main burning and re-burning are not limited to the above conditions, and the optimum burning temperature, heating rate, retention time and cooling rate may be selected according to the type (composition system) of the piezoelectric ceramic material.
Besides the lead zirconate titanate (PZT)-type compound, the piezoelectric ceramic material contained in the ceramic green sheets may contain a lead titanate (PbTiO3) compound, a lead metaniobate (PbNb2O6) compound, a lead metatantalate (PbTa2O6) compound, or the like.
The conductor contained in the conductive paste for internal electrodes is composed of Ag as the main component, and may contain up to about 50% by weight of Pd.
The atmosphere having the oxygen concentration of about 10% by volume or less can be an atmosphere containing nitrogen as a main component.
Besides nitrogen, the atmosphere gas may contain an inert gas such as argon, carbon dioxide or the like to an extent causing no adverse effect on the piezoelectric characteristics.
The main burning and re-burning steps are preferably respectively performed by separate burning apparatuses. This is because the atmosphere in each burning step can be precisely controlled.
In the method of manufacturing the monolithic piezoelectric ceramic device of the present invention where the monolithic piezoelectric ceramic device comprising internal electrodes each containing an alloy composed of Ag as the main component and formed through ceramic layers, therefore, the internal electrodes and the ceramic layers are simultaneously burned, and then again burned in the atmosphere in which the oxygen concentration is about 10% by volume or less. Ag taken in ceramic grains in the main burning step precipitates in the grain boundaries in the re-burning step to improve the piezoelectric characteristics of the device, which deteriorate due to Ag uptake, thereby improving the piezoelectric characteristics, particularly a piezoelectric strain constant (d), and sufficiently securing high reliability.
Therefore, a monolithic piezoelectric ceramic device further excellent in the piezoelectric characteristics and reliability can be obtained.